1. Field of the Invention
The present invention relates to a system for controlling the pasteurization heat treatment, in particular of packaged food products, in tunnel-type pasteurizers.
2. Description of the Prior Art
As is known, the pasteurization of packaged food products consists in the heat treatment which some types of products already packaged in final containers undergo, in order to improve storage thereof over time.
The type of pasteurization which will be considered hereinbelow is "low-temperature pasteurization" and with specific reference to food products consisting of beverages. This means that it is performed at a temperature of lower than 90.degree. C. by means of hot water which is sprayed in a programmed manner onto the containers in order to modify their temperature in accordance with a given heat cycle.
The apparatus by means of which the pasteurization process is performed essentially consists of a tunnel through which the product already packaged in the container (bottle, can or other container) is treated. From the thermal point of view, the tunnel is essentially subdivided into three zones: a first subheating zone (zone 1) where the temperature of the product is increased up to a value beyond which the actual heat treatment takes place; a second heat treatment zone (zone 2); a third cooling zone (zone 3) where the product is brought back down to about room temperature in order to avoid undesirable fermentation phenomena.
Each of these three zones is further subdivided into two or more parts for the purpose of: (a) avoiding sudden jumps in temperature and allowing the possibility of thermal recovery between the heating and cooling zones, or (b) being able to provide zones with different temperatures which are variable in accordance with a given logic, in the portion of the tunnel where the actual heat treatment occurs.
As previously mentioned, during pasteurization the product is initially heated to a certain temperature (zone 1), is then further heated up to the pasteurization temperature and kept at this pasteurization temperature for a certain predefined period of time (zone 2) and finally is cooled to a temperature close to room temperature (zone 3).
The problem arises when, for certain reasons, the product stops inside the tunnel (for example because the conveyor stops). In this case, the correct time/temperature ratio is no longer present and the product could then undergo heat treatment which is not as programmed.
Traditionally this problem is solved by regulating the temperature of the water in zone 2.
When the pasteurizer stops, a system for cooling the sprinkler water in zone 2 is automatically activated, as is known, with the aim of cooling as rapidly as possible the product to a temperature of about 50.degree. C. At this temperature, in fact, no bactericidal effect is obtained (conventionally) and the product does not undergo any alteration from an organoleptic point of view.
When the conveyor starts to move again inside the pasteurizer tunnel, the temperature in zone 2 is restored equally rapidly, in order to resume the heat treatment interrupted previously.
Obviously, in this way it is not possible to avoid the negative effects of the fluctuations, because, no matter how rapid the variations in temperature of the sprinkler water, the temperature of the product is unable to vary with the same rapidity (on account of the thermal inertia).
The outcome, therefore, is a heat cycle which is not very uniform, but which is nevertheless able to allow the product to accumulate a sufficient number of PU's (pasteurization units).
According to that disclosed in Italian patent No. 1,197,934 (entitled "Method and device for the pasteurization of food products contained in receptacles", filed on Oct. 29, 1986 in the name of Societe Nouvelle Baele Gangloff) the pasteurization heat cycle is based on the fact that the activation of the cooling systems is not linked directly to stoppage of the conveyor inside the pasteurization tunnel, but instead on the attainment of the number of PU's accumulated by the product in a particular control point located in each of the subzones into which the heat treatment zone 2 is subdivided.
The evaluation of the number of PU's accumulated by the product at the control points is obtained by means of mathematical calculation.
The drawback of this system is that the parameter which is verified is only the number of PU's accumulated, without taking into account the fact that these PU's could have been accumulated by a product which has been kept at a temperature lower than the pasteurization temperature.
This does not ensure, consequently, that the product has actually undergone the programmed heat treatment. Basically, in accordance with the known art, the completeness of the pasteurization heat treatment is deduced from the number of PU's accumulated by the product.
No attention is given, however, to the way in which these PU's are accumulated and consequently to the drawbacks which may arise therefrom in particular in relation to correctly maintaining the organoleptic characteristics of the treated product.